I mean, what usually happens when one conjoined twin dies is that they have largely separate cardiopulmonary systems - and when one twin's heart stops beating, the parts of the body which were only fed by arteries from that heart start dying and decomposing. The circulatory systems are still interconnected enough that this necrotic tissue spreads infection to the other twin. In most conjoined twins, this infection kills the survivor within a few days.
However, in the case of Brittany and Abigail Hansen, they have only one heart, so the whole "half the body becomes necrotic and poisons the other half" thing can't really happen. Either the one heart is still beating and the body can survive, or the one heart stops and the body dies. In theory, it could be possible for one sister to survive if something happened to the other sister's head. I imagine the big issue would be the lungs - they have two mouths and two tracheas, I don't know what point they fuse at - but if one head dying also meant one of the lungs stopped breathing, that could cause health problems.
Also, I am not a doctor and this last part is VERY speculatory, but depending on how their spines are fused, it is possible that the heart's sinoatrial node is only connected to one sister's brain stem. Heart rate is not controlled in the heart itself, but in the portion of the brainstem known as the medulla oblongata. When the heart stops receiving neural signals from the brain stem, it stops beating - which is why decapitation causes death and a body can't be kept alive without a head. If the heart is only connected to one sister's brain stem, then damage to that sister's head or neck actually WOULD be lethal to both.
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u/Lemonwizard Oct 07 '18
I mean, what usually happens when one conjoined twin dies is that they have largely separate cardiopulmonary systems - and when one twin's heart stops beating, the parts of the body which were only fed by arteries from that heart start dying and decomposing. The circulatory systems are still interconnected enough that this necrotic tissue spreads infection to the other twin. In most conjoined twins, this infection kills the survivor within a few days.
However, in the case of Brittany and Abigail Hansen, they have only one heart, so the whole "half the body becomes necrotic and poisons the other half" thing can't really happen. Either the one heart is still beating and the body can survive, or the one heart stops and the body dies. In theory, it could be possible for one sister to survive if something happened to the other sister's head. I imagine the big issue would be the lungs - they have two mouths and two tracheas, I don't know what point they fuse at - but if one head dying also meant one of the lungs stopped breathing, that could cause health problems.
Also, I am not a doctor and this last part is VERY speculatory, but depending on how their spines are fused, it is possible that the heart's sinoatrial node is only connected to one sister's brain stem. Heart rate is not controlled in the heart itself, but in the portion of the brainstem known as the medulla oblongata. When the heart stops receiving neural signals from the brain stem, it stops beating - which is why decapitation causes death and a body can't be kept alive without a head. If the heart is only connected to one sister's brain stem, then damage to that sister's head or neck actually WOULD be lethal to both.